1. Field of the Invention
The present invention relates to a silicon-controlled rectifier, particularly to a bidirectional PNPN silicon-controlled rectifier, which has a small area and a high ESD resistance.
2. Description of the Related Art
With the advance of semiconductor technology, the dimensions of MOS (Metal Oxide Semiconductor) elements have been reduced to a submicron or even deep-submicron scale. The submicron or deep-submicron technology uses so thin a gate oxide layer that only a few volts higher voltage is enough to cause damage. In common conditions, the voltage of electrostatic charge may reach thousands or even several ten thousands of volts, which will damage integrated circuits (IC). Therefore, once having accumulated to a given amount, electrostatic charge should be released. The silicon-controlled rectifier, which has a low turn-on resistance, low capacitance, low power consumption and high-power current conduction capability, is exactly an effective ESD (Electro-Static Discharge) protection element for IC.
Currently, bidirectional silicon-controlled rectifiers (SCR) have become the mainstream in the market of the ESD protection circuits for I/O ports subject to both positive and negative voltage signals, and many researches are also dedicated to bidirectional silicon-controlled rectifiers. U.S. Pat. Nos. 6,258,634, 6,365,924 and 7,034,363 all disclosed symmetric bidirectional silicon-controlled rectifiers. As the silicon-controlled rectifiers disclosed in the abovementioned patents are all directly fabricated on a silicon substrate, they have lower breakdown voltages and can only apply to common IC processes. A U.S. Pat. No. 6,960,792 disclosed an annular-layout symmetric bidirectional silicon-controlled rectifier. Such a technology needs a larger layout area. Further, the trigger speed is also deeply influenced by the structure thereof. Therefore, this technology cannot provide an effective ESD protection function. A U.S. Pat. No. 5,072,273 disclosed a low trigger voltage silicon-controlled rectifier. However, it can only operate unidirectionally. A U.S. Pat. No. 7,145,187 disclosed a special silicon-controlled rectifier. However, it is hard to mass fabricate and lacks utility.
Accordingly, the present invention proposes a novel bidirectional PNPN silicon-controlled rectifier to overcome the abovementioned problems.